1. Field of the Invention
The present invention relates generally to a broadband wireless communication system, and in particular, to a method and apparatus for performing fast handover using fast ranging.
2. Description of the Related Art
A broadband wireless access communication system which is now under discussion in an Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group performs point-to-multipoint communication between a base station (BS) and a subscriber station (SS). A physical (PHY) layer standard defines Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD) as a duplexing scheme, and Time Division Multiplexing using Single Carrier (TDM-SC), Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) as a multiplexing scheme, and defines a Medium Access Control (MAC) layer standard capable of operating in common in the foregoing PHY standards.
With reference to FIG. 1, a description will now be made of a conventional communication system configuration taken into consideration in IEEE 802.16.
FIG. 1 is a diagram schematically illustrating a configuration of a broadband wireless access communication system introducing the cellular concept, and in particular, a configuration of an IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system is based on a cellular configuration, and is comprised of a BS#1 110 and a BS#2 130, each of which manages its own cell, a plurality of SSs 120a, 120b, 120c and 120d managed by the BS#1 110, and a plurality of SSs 140a, 140b and 140c managed by the BS#2 130. The SSs are classified into fixed SSs (FSS) and mobile SSs (MSS) according to their mobility.
A radio link 150 between the BSs 110 and 130 and the SSs 120a, 120b, 120c, 120d, 140a, 140b and 140c, through which signals are transmitted/received, is realized using the foregoing PHY schemes. The BSs 110 and 130 are connected to each other using a wire, for information exchange therebetween.
If the MSS #4 120d is moving to an overlapping area between the cells managed by the BS#1 110 and the BS#2 130 and continuously moves from the BS#1 110 currently serving the MSS#4 120d (called a serving BS) toward a BS#2 targeted by the MSS#4 120d (called a target BS), then handoff or handover will occur. That is, a serving BS of the MSS#4 120d is changed from the BS#1 110 to the BS#2 130.
FIG. 2 is a diagram illustrating a frame structure of a TDD OFDMA system, an example of a broadband wireless access communication system.
Referring to FIG. 2, a horizontal axis represents an OFDM symbol number, and a vertical axis represents a subchannel number. As illustrated in FIG. 2, each OFDMA frame includes a downlink (DL) subframe comprised of a plurality of, for example, 6 OFDM symbols, and an uplink (UL) subframe comprised of a plurality of, for example, 5 OFDM symbols. Each of the OFDM symbols is comprised of a plurality of, for example, M subchannels.
Each of the TDD OFDMA frames has DL-MAP 210 and UL-MAP 220 representing resource allocation information of downlink/uplink subframes. The DL-MAP message indicates how the resources constituting a downlink subframe are allocated to SSs, and the UL-MAP message indicates how the resources constituting an uplink subframe are allocated to the SSs.
The TDD OFDMA frame may include a Downlink Channel Descriptor (DCD) message 230a, an Uplink Channel Descriptor (UCD) message 230b, and a Neighbor Advertisement (NBR-ADV) message 230c, and those messages are periodically included in the TDD OFDMA frame and can be different from each other in terms of a reception period. The DCD message 230a includes downlink channel-related parameters, the UCD message 230b includes uplink channel-related parameters, and the NBR-ADV message 230c includes information on neighbor BSs.
FIG. 3 is a diagram illustrating an initial ranging procedure for compensating for a round trip delay (RTD) due to a position difference between a BS and an SS in a broadband wireless access communication system. Referring to FIG. 3, a BS 310 allocates an initial ranging interval 311 corresponding to a multiple of an initial ranging transmission opportunity capable of accepting an RTD of an SS#n 330 located in the farthest position from its cell coverage. In FIG. 3, the BS 310 includes one initial ranging transmission opportunity. After allocating the initial ranging interval, the BS 310 broadcasts its information to all of the SSs through a UL-MAP.
SSs that should perform initial ranging, e.g., an SS#1 320 and an SS#n 330, transmit Ranging Request (RNG-REQ) messages 321 and 331, respectively, at a start time of the initial ranging interval 311, designated by the UL-MAP. In an OFDMA scheme, the RNG-REQ message includes a CDMA ranging code.
The RNG-REQ messages 321 and 331 are transmitted on a competition basis, and the competition-based transmission may cause message collision for SSs located the same distance from the BS 310. To address this problem, the IEEE 802.16e communication system standard allows for the SSs to randomly determine transmission opportunities before transmitting the RNG-REQ messages. The OFDMA scheme allows for the SSs to randomly select before transmission not only the transmission opportunities but also the ranging codes from a specific set, thereby reducing message collision. However, the OFDMA scheme still suffers from the message collision.
The SS#1 320 and the SS#n 330 do not suffer RNG-REQ collision due to their different distances from the BS 310. Therefore, the BS 310 can successfully receive the transmitted RNG-REQ messages. The BS 310 can measure an RTD value 312a of the SS#1 320 by calculating a time difference between a reception time of the RNG-REQ message 321 transmitted from the SS#1 320 and the start time of the initial ranging interval 311, and can measure an RTD value 312b of the SS#n 330 by calculating a time difference between a reception time of the RNG-REQ message 331 transmitted from the SS#n 330 and the start time of the initial ranging interval 311.
The BS 310 permits the SSs 320 and 330 to adjust their uplink transmission times by supplying the measured RTD values to the SS#1 320 and the SS#n 330 through the Ranging Response (RNG-RSP) messages 322 and 332. The foregoing processes are repeated until the uplink transmission times of the SSs 320 and 330 arrive at a range specified by the BS 310. As the BS 310 allocates uplink resources to the SS#1 320 and the SS#n 330, the following RNG-REQ transmission can be achieved on a non-competition basis.
FIG. 4 is a flowchart illustrating an initial network entry and handover procedure of an MSS according to the IEEE 802.16e communication system standard. Referring to FIG. 4, upon power-on, the MSS first performs a cell selection process (Step 401). The cell selection process is a quality measurement process for uplink/downlink channels, and includes the process of receiving DL/UL-MAP messages and DCD/UCD messages for the downlink, and the initial ranging process for the uplink. In the cell selection process, the MSS records the collected information on a plurality of cells for future use, selects a cell providing the best uplink/downlink quality from among the cells, and performs a network entry process described below according to the cell selection result.
After completion of the cell selection, the MSS performs a process of synchronizing to a downlink provided by a BS of the selected cell and acquiring reception parameters (Step 403). The parameter acquisition process is comprised of a process of continuously receiving DL-MAP messages and receiving their associated DCD messages. After the downlink synchronization, the MSS should receive an UCD message from the BS in order to acquire possible transmission parameters for an uplink channel (Step 405).
After acquiring the uplink parameters, the MSS adjusts its uplink transmission parameters such as time offset, frequency offset and power offset through an initial ranging procedure (Step 407). During the initial ranging procedure, the MSS is allocated, from a BS, a connection identifier (CID) to be used later for the transmission/reception of a control message.
Upon completion of the initial ranging, the MSS supplies its traffic transmission/reception capabilities to the BS, and the BS supplies the following MSS-BS traffic transmission/reception capabilities to the MSS through a message, based on information on the MSS and its transmission/reception capabilities, thereby performing a basic capacity negotiation procedure (Step 409).
After the basic capacity negotiation procedure, the MSS should perform authorization and key exchange with the BS according to a procedure specified in the IEEE 802.16 standardization (Step 411). After completion of the authorization and key exchange, the MSS is allocated from the BS an additional CID for the control purpose and registers with the BS (Step 413). After completion of the registration with the BS, the MSS is allocated an Internet Protocol (IP) address for traffic transmission/reception during an IP connection setup process (Step 415), and performs a process of setting a system time and acquiring system operation parameters (Step 417). Subsequently, the MSS is allocated an additional CID to be used for traffic transmission/reception for each service flow (Step 419), and then ends a network entry procedure if it arrives at a normal mode for traffic transmission/reception (Step 423).
In the normal mode, an MSS that can transmit/receive traffic needs to perform periodic ranging at intervals of the time negotiated with the BS in order to acquire uplink synchronization and maintain/correct transmission parameters. In addition, the MSS should acquire a network topology with the assistance of the BS (Step 421). This enables a faster network re-entry process during handover. The network topology acquisition (Step 421) is achieved through periodic broadcasting by the BS of information of neighbor BSs. Here, the broadcasting by the BS of information of neighbor BSs is achieved through the transmission of an NBR-ADV message.
If a level of a downlink signal transmitted from the BS, i.e. a serving BS, drops below a specified threshold, the MSS searches for a BS to serve as a new serving BS, i.e. a target BS, using the information of the neighbor BSs acquired through the NBR-ADV message. At this point, the MSS can only measure the levels of the downlink signals from the candidate target BSs, or transmit the RNG-REQ messages to the candidate target BSs along with the level measurement on the downlink signals. In the following description, the former case where the MSS only measures the levels of the downlink signals from the target BSs will be referred to as “passive scanning,” while the latter case where the MSS performs both the downlink signal level measurement and the RNG-REQ message transmission will be referred to as “active scanning.”
The candidate target BS receiving the RNG-REQ message transmitted through the active scanning provides the MSS with an uplink parameter adjustment value and an estimated service level through the RNG-RSP transmission. When a downlink signal level of the serving BS is less than a signal level of a candidate target BS collected through the scanning process, the MSS transmits a Handover Request (HO-REQ) message to the serving BS to thereby start a handover process (Step 425).
The HO-REQ message can include information related to a plurality of candidate target BSs. The serving BS receiving the HO-REQ message selects the best target BS through the information exchange with the candidate target BSs, and notifies the MSS of the selected best target BS through a Handover Response (HO-RSP) message. The MSS receiving the HO-RSP message sends a Handover Indication (HO-IND) message to the serving BS, and the serving BS withdraws all of the system resources allocated to the MSS upon receipt of the HO-IND message (Step 427).
The MSS begins a network re-entry process to the target BS starting with the process of synchronizing to a downlink from the target BS and acquiring related parameters (Step 431). Subsequently, the MSS performs an uplink parameters adjustment process (Step 437) through a uplink parameter acquisition procedure (Step 433) and a ranging procedure (Step 435).
After successfully adjusting the uplink parameters, the MSS performs an authorization process with a new serving BS (Step 439), and sets up a connection with a MAC layer by performing a registration process with the new serving BS (Step 441). By doing so, the MSS can normally perform data transmission/reception with the new serving BS, and can be allocated a new IP address in the following process (Step 443).
As described above, in the conventional technology, a BS can assign a CID to a corresponding SS while transmitting an RNG-RSP message in response to an RNG-REQ message, and can allocate uplink resources for the following non-competition-based transmission of RNG-REQ message. At this point, such resources are not necessary for an SS aiming at cell selection and topology acquisition.